The present invention relates to a photoresist that can be developed in an aqueous alkaline solution, for generating structures in the sub-.mu.m range. The photoresist is composed of a developable base polymer, a photo-active component, and, as desired, additives.
Positive photoresists, that can be developed in an aqueous-alkaline solution, are presently used for the generation of photolithographic structures, and the transfer of a structure from a master (mask) onto a substrate, for example onto a wafer. These photoresists are essentially composed of an alkali-soluble base polymer and a photo-active component (PAC).
These photoresists function by what can be characterized as the solution inhibitor principle. The presence of a hydrophobic PAC in the photoresist effectively reduces the solubility of the base polymer in the aqueous-alkaline developer. It is only when the photoresist is irradiated with light having a suitable wavelength that the PAC is converted, through a photochemical process, into a hydrophilic compound. Once the PAC is so converted, the polar and, thus, hydrophilic groups, become available in the exposed regions, and are soluble to the developer. In this process, the base polymer is chemically dissolved by the developer, only in those exposed regions that are accessible to the developer.
Typical photoresists can be adapted to photolithography processes through the use of light in the visible range (for example, 436 nm) or light in the near ultraviolet range (for example, 365 nm). However, using these photoresist systems, only structures up to about 0.5 .mu.m can be optically resolved and therefore, produced. According to the formula ##EQU1## the smallest imaged structure CD is proportional to the wavelength .lambda. used for the exposure. In the formula, NA is the numerical aperture and represents a parameter of the optics used for the imaging, whereas k is a processed-associated factor.
In order to further increase the packing density of integrated circuits or memory modules in micro-electronics, it is necessary to reduce the dimensions of the structure. This can be accomplished, for example, through photolithography by reducing the wavelength of the light used for the imaging. Special resist systems, however, are necessary for the use of light in the deep ultraviolet light (DUV) having wavelengths that are below 300 nm. Specifically, a base polymer that is transparent to this wavelength range is necessary in addition to a PAC that "decomposes" in deep ultraviolet light. Whereas the PAC should initially have a high absorption in order to ensure a high sensitivity, it must bleach out during the course of the imaging exposure, i.e., it must become transparent again, with respect to the radiation that is used to expose after the photochemical reaction. The light can penetrate more deeply into the resist layer and the resolution, the contrast, and the edge steepness of the structures produced in this manner are greater the lower the residual absorption of the photoresist.
Although there are known resist systems that are adapted for use in the near ultraviolet region (NUV), these, however, exhibit a high residual absorption of the base polymers in the DUV. Moreover, they lose their desirable properties upon exposure to a wavelength of, for example, 248 nm (KrF excimer laser as light source). This results in poor values for contrast, sensitivity, resolution, and edge steepness of the resultant structures. It is therefore necessary to develop new resists for use in DUV lithography.
Reducing the wavelength of the light, likewise, reduces the depth of focus (DOF) and creates a further problem when a shorter-wavelength light is used. The behavior of the DOF can be expressed in the formula: ##EQU2##
Although the contrast sharpness of imaged structures is more readily ensured using thin resist layers, thick resist layers are required, particularly for substrates that use topography steps. The thick layers are required, for example, in order to achieve a planarization.
To address this problem, a two-layer technique is used wherein an optically dense, planarizing, first resist layer is initially applied on the substrate and is subsequently covered with a second, thin, photolithographically structurable resist layer. In the structuring that follows, it is only this second resist layer that is initially exposed and developed. The structure that is created then serves as a mask for structuring the first resist layer through an anisotropic etching process, for example, with oxygen-containing plasma.
This technique compensates for the unevenness of the substrate and also prevents the generation of disruptive reflections from the substrate surface during the exposure. However, it is necessary that the second structurable resist layers, referred to as the top resist, have an adequate etching resistance with respect to, for example, oxygen plasma. To provide this etching resistance, certain elements are added to the base polymer, for example, silicon. Non-volatile oxides, that protect the top resist structures against further etching, are formed from these elements in the oxygen plasma.
An article by T. R. Pampalone in the periodical, Solid State Technology, June 1984, page 115 suggests the use of cresol-novolaks as a base polymer for DUV-compatible photoresists. Cresol-novolaks are known for their etching resistance in halogen plasma. Cresol-novolaks are used as the base polymer for longer-wavelength lithography processes in commercial photoresists. Even though the use of an increasing proportion of para-cresol in the novolak present as an isomer mix, improves the transmission for light in the deep ultraviolet light range, it cannot be reduced below an absorption coefficient .alpha..sub.248 of 0.3 .mu.m.sup.-1. This absorption value limits the applicability of cresol-novolaks in DUV photoresists.
The Pampalone article also proposes poly (4-hydroxystyrol) for NUV and DUV photoresists. The poly (4-hydroxystyrol) exhibits an etching resistance comparable to novolaks, but, due to its high alkali solubility, has unacceptably high dark erosion rates in the development of resist mixtures. Therefore, it is unsuitable as a photoresist for sub-.mu.m structures.
C. E. Osuch et al in SPIE Vol. 631, Advances in Resist Technology and Processing III (1986), pages 68-75 describes a further photoresist system for the DUV range. The photoresist system described makes use of a styrol-maleic acid imide copolymer having tertiary butoxy-carbonyl units, that block the imide groups, as a base polymer. The base polymer is used in combination with an acid-forming, photo-active component, in an alkali-soluble photoresist that is structurable by light in the DUV range.
This photoresist system, however, has the disadvantage that it does not bleach during exposure. Therefore, the system is limited to relatively thin film layers. Moreover, the photoresist system requires a developing process that includes an additional heating step that results in a loss of approximately 10% of the layer thickness and makes correspondingly thicker resist layers necessary. As a result thereof, structures of only approximately 0.75 .mu.m can be resolved with this system (given a layer thickness of 1.25 .mu.m).
Further DUV photoresists have been proposed for use in a two-layer resist technique. For example, in one system, a copolymer of trimethylsilylmethyl-methylacrylate and methylacrylic acid in a molar ratio of 1:1 is used as an alkali-soluble base polymer with 2-nitrobenzyl-cholate as a photoactive component for DUV top resists that contain silicon. This system, however, has a relatively low silicon content of about 8%, a low photosensitivity of 200 mJ/cm.sup.2, and a maximum resolution of only about 0.75 .mu.m. Accordingly, the applications of this system are also limited.
With respect to the trimethylsilyl groups in the alkali-insoluble base polymer poly (4-trimethylsiloxy-styrol), these can be split off by using a strong acid upon the formation of phenol groups. By using an onium salt as a photo-active component, the polymer is therefore suitable as a DUV resist, but only in limited fashion. However, the polymer does not bleach out during the exposure and therefore has a contrast of only about 1.2. Moreover, the base polymer has a low softening point of approximately 76.degree. C. that prevents its use as a top resist for an oxygen plasma step, wherein temperatures of approximately 100.degree. C. are reached.